1. Field of the Invention
The invention relates to pulse width modulated switching amplifiers for analog signals and in particular to means for maximizing the efficiency and linearity of such amplifiers.
2. Description of the Prior Art
Pulse width modulated switching amplifiers are used for high power amplification of analog signals due to the high efficiency realizable when low energy dissipation switching devices are used. Generally, these amplifiers operate at a switching frequency which is high with respect to the highest frequency of the input signals to be amplified, and employ either two-state or three-state switching circuits to produce a pulse width modulated waveform. This waveform is then demodulated, usually by a simple low-pass filter, to yield the desired amplified signal output.
A two-state switching amplifier typically produces a constant frequency output waveform which, before demodulation, cyclically alternates between fixed positive and negative magnitudes. The relative durations of the positive and negative pulses (forming the positive and negative portions of each cycle of this output waveform) vary in response to the instantaneous magnitude of the analog signal applied to the input of the amplifier. As the magnitude of the analog signal approaches the upper limit of the operating range of the amplifier, the output waveform remains at the polarity of the analog signal much longer than it remains at the opposite polarity during each cycle. As the instantaneous magnitude of the analog signal decreases to zero, the durations of the positive and negative pulses forming each cycle of the output waveform approach equality.
The two-state switching amplifier has the advantage of high linearity over its entire operating range, but suffers from the disadvantage that its output signal contains a spurious signal in the form of undesirably large spectral lines at the switching frequency. Also, its efficiency generally is somewhat lower than that of the three-state switching amplifier.
A conventional three-state switching amplifier also produces cyclically occurring pulses of fixed magnitude and usually of fixed periodicity, although in some cases the period may be made variable. A pulse is produced during eacy cycle, with the same polarity as the analog signal applied to the input of the amplifier. The duration of each pulse relative to the period of the cycle in which it occurs is determined by the instantaneous magnitude of the analog signal. As the magnitude of the analog signal approaches the upper limit of the operating range of the amplifier, it produces pulses of the same polarity as the analog signal and of a duration approaching the full period of one cycle. During the remainder of each cycle no output signal is produced. As the magnitude of the analog signal decreases to zero, the duration of the pulses decreases to zero.
The three-state switching amplifier has the advantage of a low spurious output relative to the desired signal, but suffers from the disadvantage of poor linearity at the lower end of the operating range. As the magnitude of analog signals applied to the input of the amplifier approaches zero, the durations of the pulses required to represent such signals approach the storage time of the amplifier switching transistors.
It is the primary object of the present invention to provide a pulse width modulated switching amplifier having the linear operating range of a two-state switching amplifier, and producing signals having the low spurious signal content characteristic of three-state switching amplifiers.